1. Field of the Invention
The present invention relates to a toasting-time controller for a toaster and, more particularly, to a toasting-time controller adapted to control the energizing time of the toasting heater of a toaster by the action of a bimetal installed externally of the toasting chamber of the toaster.
2. Description of the Prior Art
Many types of toasters, adapted to automatically control energizing of the toasting heater so as to toast slices of bread to a desired degree of browning, have been heretofore put into practical use. For instance, there are toasters of the type adapted to make the toasting time constant. The toaster of this type has a mechanical timer, which starts operation simultaneously when the toasting heater begins to be energized and which stops operation to deenergize the toasting heater when a predetermined time set thereby has elapsed. Thus the toaster of this type has advantages in that the energizing time of the toasting heater is very accurate relative to its set value and in that it can securely hop up slices of toast. In such a toaster, however, the energizing time of the toasting heater is set totally independently of the power supply voltage and ambient temperature; therefore the temperature of sliced bread just after toasting is not necessarily constant, however accurate the above-mentioned energizing time may be determined. In other words, such factors as power source voltage and ambient temperature must be taken into consideration in addition to time to toast slices of bread to a constant degree of browning; accordingly it is desirable that the above energizing time should be changed according to changes in power source voltage and ambient temperature. For this reason, the toaster of the above-mentioned type which controls only the above energizing time cannot toast slices of bread uniformly.
In view of the above facts, there have been proposed toasters of the type which control the above energizing time in consideration of the power source voltage and ambient temperature. Of such toasters, those of which are called the "internal bimetal" type have an energizing-time controlling bimetal installed in the toasting chamber in which the toasting heater is provided. In the toaster of this type, the bimetal is deformed under the influence of the temperature inside the toasting chamber elevated by heat generated from the toasting heater; thus the bimetal may deform in response to changes in power source voltage and ambient temperature and thereby the energizing time of the toasting heater may be determined in consideration of changes in power source voltage and ambient temperature; as a result, the toaster of this type can advantageously toast slices of bread substantially uniformly. However, the toaster of this type has a fatal disadvantage in that it cannot carry out a second toasting immediately after a first toasting is completed because the bimetal once bent in the first toasting cannot return to its original state immediately. In order to perform toasting operations successively, it is necessary to provide an interval between two successive toasting operations so as to sufficiently lower the temperature inside the toasting chamber. In addition, the toaster of this type has another disadvantage in that the bimetal must be set in the vicinity of the surface of sliced bread to be browned and therefore prevents radiant heat partially from reaching the sliced bread with the result that the sliced bread is unevenly browned and accordingly becomes poor in appearance. In the toaster of the above-mentioned internal bimetal type, which detects elevation of the temperature in the toasting chamber, there also exists a disadvantage in that the bimetal must be set at a predetermined position and therefore the time setting for the toaster tends to greatly vary with the loading condition. For instance, the toaster of the type capable of toasting up to two slices of bread at one time can correctly set the toasting time when two slices of bread are placed therein for toasting; if, however, only one slice of bread is placed therein for toasting, heat supplied to the bimetal greatly varies with the position of the slice of bread and therefore the toasting time becomes inaccurate with the result that the slice of bread cannot be browned to a desired degree. In terms of fabrication, such a toaster also has a disadvantage in that it requires adjustment of the toasting time after assembly since it cannot provide a proper toasting time by the assembly alone; in addition, the adjustment requires a very long time since it must use proper loads, for instance, slices of bread or imitation bread made of asbestos or the like to ensure high accuracy.
In view of the above disadvantages of the toaster of the internal bimetal type, there is also provided a toaster of the external bimetal type in which the bimetal is set externally of the toasting chamber and is adapted to be heated by a heater provided separately from the toasting heater. As in the case of the internal-bimetal type toaster, the toaster of the external bimetal type can set the toasting time in consideration of the power source voltage and ambient temperature. In addition, immediately after one toasting operation is completed, it can perform setting of the bimetal for the next toasting operation.
However, the deformation of the bimetal is not proportionally related to time, the deformation rate being the largest at the start of energizing and becoming smaller with the lapse of time; therefore, the bimetal must be so installed that the interval between its switching-on operation after heating and its switching-off operation after cooling may correspond to the range where the deformation rate is small and, as a result, correct on-off operation can hardly be expected. Especially important is the fact that the deformation of the bimetal is related to the power source voltage, the variation of time relative to that of the power source voltage being too great to stabilize the operating points. Besides, the above-mentioned features of the bimetal lead to the results that the shorter the toasting time, the higher the time accuracy and the longer the toasting time, the lower the time accuracy becomes and therefore, the toasting time cannot be lengthened beyond a certain extent. Such a time innaccuracy problem is aggravated as the output of the toasting heater gets lower; and thus, it is generally impossible to set a long toasting time for a low-output toaster.